Devices called fiber optic rotary joints allow optical signals to be transferred between fibers located on rotating and stationary members. The device is categorized as an on-axis rotary joint when the fibers are located along the axis of rotation. The device is categorized as an off-axis rotary joint if access to the axis of rotation or centerline is not possible. The technology employed in these two types of rotary joints is quite different. The present invention concerns off-axis rotary joints.
Contactless fiber optic rotary off-axis joints have been developed as disclosed in U.S. Pat. No. 4,525,025 to the present assignee. The '025 patent discloses a fiber optic rotary joint which couples a pulsed optical signal across a rotary interface and includes an annular reflective wall formed on a stator and an optic fiber mounted on the stator having one end in close proximity and tangential to the annular reflective wall. A signal emitted by one of the optic fibers will be reflected along the annular reflective wall and received by the other of the optic fibers.
Actual joints constructed in a manner similar to that generally disclosed in the '025 patent have been limited to a rotor diameter of 10-12 inches and data rates of 50 megabits/sec. due to unacceptable propagation delays causing bit pulse-width distortion. There is a need for joints having rotor diameters of 40-50 inches using pulsed optical signals having data transfer rates of 100-400 megabits/sec. To meet these requirements, two criteria must be met. First, optical variations with rotation must be minimized. Second, propagation delays must be controlled to minimize effect on bit pulse-width distortion.
Optical variations with rotation can be minimized by using a multiplicity of optical pick-ups spaced circumferentially. The problem is that it is desirable to have as few pick-ups as possible to minimize complexity and cost.
Propagation delays must be controlled. For example, consider a waveguide that is formed into a continuous 360.degree. arc that is four meters in circumference. If four fiber optic pick-ups located equidistant around the circumference are focused to a common photodiode and a single light source is used to inject a signal into a waveguide at a point of injection, then the optical pick-up that is nearest to the point of injection will receive the transmitted signal first and thereby transmit the received signal to the photodiode first. Because the second optical pick-up is located 90.degree. away, the optical signal travelling from the point of injection at a speed of three ns/meter will arrive at the second pick-up three ns after the first. Similarly, the third pick-up would receive the transmitted signal after nine ns. Thus, for a four meter circumference continuous waveguide, a propagation delay of twelve ns would result. For a 100 Mb/s signal, which has a 10 ns bit width to be transmitted under these conditions, the bit shape would be distorted by signals arriving at the different optical pick-ups at different times because the propagation delays are larger than the bit width. A larger diameter joint exacerbates the problem and would exhibit even larger delays.
There is at least one arrangement currently being suggested that claims to achieve the previously mentioned circumference and data rate requirements. This suggested arrangement uses a plurality of short pieces of optical fiber arranged circumferentially in a ring shape on a stator to form an optical waveguide. A corresponding plurality of receivers or pick-ups are optically coupled to a respective short piece of fiber. A corresponding plurality of laser transmitters are circumferentially arranged on a rotor and transmit optical signals into the waveguide with each short piece of optical fiber receiving an optical signal from one of the transmitters.
This proposed arrangement has several disadvantages. Because the optical signal is being transferred in a fiber optic medium, the propagation speed of the fiber optic medium ultimately limits the effective length of the waveguide as the bit-rate increases. Thus, the length of the short pieces of fiber must be reduced as the data rate increases. This reduction in length requires more transmitters and receivers adding to cost and complexity. Additionally, the losses in the fiber optic medium requires a more powerful laser transmitter and/or a more sensitive receiver.